The commercial trucking industry is a large market in need of automatic tire inflation systems. Tractor and trailer rigs can typically have 18 tires, and because of their constant use and cost it can be justifiable for a fleet to purchase tire inflation systems for tire pressure maintenance. Properly inflated tires can greatly extend tire life and reduce hazardous blowouts. To meet the needs of the trucking industry, a low cost, low maintenance, and durable rotary union is essential.
A typical rotary union for such a system has one member fixed in relation to the end of the axle and another fixed in relation to the hub cap. Some will incorporate a smooth graphite face seal to serve as the dynamic wear seal and others use elastomer seals. Most existing designs have relatively large dynamic sealing surfaces and their size is proportional to the surface speed of the wearing surface. Large seals can compensate for misalignment between the axle and hub cap since the large cross-section will expand and contract with every half rotation. A larger seal helps guarantee sealing but generates more heat, costs more, and generally wears more quickly. Some tire inflation systems add expensive controls to provide air only momentarily to the rotary unions to extend their life, limiting the effectiveness of the inflation system.
As shown in U.S. Pat. No. 5,769,979, the rotary union is able to use a small dynamic seal by compensating for bearing wobble and misalignments between the hub and axle through the use of a pivoting rigid tube extending between two elastomer seals. Distortion of the elastomer seals while the tube pivots can impair their sealing ability and longevity. Providing sufficient clearance around the seals so the rigid tube can pivot freely can also be a detriment to a seal's performance since proper backing isn't provided for the seal. A rigid tube also is vulnerable to permanent bending during handling and installation.